Ultrastructure of the antennal sensilla of the praying mantis Creobroter nebulosa Zheng (Mantedea: Hymenopodidae)

The praying mantis Creobroter nebulosa Zheng (Mantedea: Hymenopodidae) is an insect that has medicinal and esthetical importance, and being a natural enemy for many insects, the species is used as a biological control agent. In this publication, we used scanning electron microscopy (SEM) to study the fine morphology of antennae of males and females of this species. The antennae of both sexes are filiform and consist of three parts: scape, pedicel, and flagellum (differing in the number of segments). Based on the external morphology and the sensilla distribution, the antennal flagellum is could be divided into five regions. Seven sensilla types and eleven subtypes of sensilla were observed: grooved peg sensillum (Sgp), Bohm bristles (Bb), basiconic sensillum (Sb), trichoid sensillum (StI, StII), campaniform sensillum (Sca), chaetic sensillum (ScI, ScII, ScIII), and coeloconic sensillum (ScoI, ScoII). In Mantodea, the ScoII is observed for the first time, and it is located on the tip of the flagellum. The external structure and distribution of these sensilla are compared to those of other insects and possible functions of the antennal sensilla are discussed. The males and females of the mantis could be distinguished by the length of antennae and number of Sgp. Males have antennae about 1.5 times longer and have significantly larger number of Sgp compared to females. The sexual difference in distribution of the Sgp suggests that this type of sensilla may play a role in sex-pheromones detection in mantis.

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Introduction
The antennae of insects are highly precise sensory structures, which are important sensory organs involved in receiving various environmental signals, the sensilla on the antennae include mechanical sensilla, chemical sensilla, thermo-and hygro-receptive sensilla [1][2][3].The sensilla play an important Ultrastructure of the antennal sensilla in the praying mantis Creobroter nebulosa Zheng (Mantedea Hymenopodidae) Click here to access/download;Manuscript;wyang369.pdf role in insect life activity, such as habitat seeking, food searching and recognition of sexual partner location [4].Previous studies have shown that the type and distribution of sensilla in antennae are related to sex, foraging and courtship, which are decided at the stage of insect morphogenesis [5].
Structurally, an antenna organization with a scapus, pedicellus, and flagellum, which is divided into segments (flagellomeres) [6].Sensilla are the basic structures that allow the perception of environmental signals that play a role as mechanoreceptors, chemoreceptors, thermoreceptors and hygroreceptors associated with their morphology [7].Sensory organs that are distributed on antennae surface and their shapes are different, because of their different functions [7], among other functions, some of these sensilla conduct olfactory information coming from odors or pheromones [8].Sensilla demonstrate different shapes, being classified into grooved peg sensilla, basiconic sensilla, trichoid sensilla, coeloconic sensilla, campaniform sensilla and chaetica sensilla with variations in these main types [9][10][11].
Mantodea is a predatory insect group including about 3000 species with moderate diversity [12].
They vary considerably in their ecology, ranging from cursorial hunters to sit-and-wait predators and live in a wide variety of environments [13].To detect and attack prey, it is equipped with binocular vision using a pair of compound eyes [14].And within the Mantodea, both chemical and olfactory cues are also important [15].Some mantids exhibit characteristic behavioural displays involving colourful patterns on the raptorial fore-legs, wings, and thorax [16].
Tenodera aridifolia Stoll (Mantodea: Mantidae) is a widely distributed and important predatory insect.Previous studies have described the cuticular fine structure of the antenna in T. aridifolia based on scanning electron microscope images, and explored the relationship between antennal morphology and behavior [17][18][19].Focal issues are the structure of scape, pedicel, basiflagellum, and distiflagellum with sensilla, the basal articulations of the antenna, and the surface sculpture of the basiflagellomeres [6].There are six types of sensilla on the antenna of the T. aridifolia, including grooved peg sensilla, basiconic sensilla, trichoid sensilla, campaniform sensilla, chaetic sensilla and coelocapitular sensilla [19].Carle, Yamawaki et al. developed a new and innovative methodology to reconstruct the antennal development model based on the length of flagellomeres [18].The morphology of male antennae is related to the competition between praying mantis, for example, the males who had more trichoid sensilla located females quicker than did males with fewer trichoid sensilla [3].Carle and Horiwaki et al. using praying mantises (T.aridifolia) as an example of a sit-and-wait predator, they examined the effects of conspicuousness and novelty of prey on avoidance learning [2].
The aim of the present study was to provide the detailed fine morphological characterization of the antennae of C. nebulosa using light and scanning electron microscopy (SEM), the fine structure, location and distribution of different sensilla types e in both males and females were investigated to record differences between the sexes.The difference of antennal sensilla between C. nebulosa and T. aridifolia was discussed and the reasons for these differences were speculated.These data provide an important starting point, they are complementary to further sensory function and behavioural studies on C. nebulosa and other Mantodea species, and may be useful for the future study of taxonomic and phylogenetic analysis of Mantodea.

Study species
Male and female adult mantises (C.nebulosa) were used in this experiment.The experimental materials were raised in captivity.

Optical microscope
Antennae of adults were observed using an optical microscope (Nikon, SMZ745T, Japan).
Micrographs of flagellomeres antennae were taken along antennae.

Scanning Electron Microscopy
To identify the sensilla types and describe the antennal ultrastructure.First, antennae were removed by cutting the cuticle of the head under the optical microscope, and cleaned using an ultrasonic cleaner (KQ-250DM, Supmile, kunshan, China) for 5 mins to wash away the surface impurities.Then transferred them with 2.5% glutaraldehyde solution and fixed for 2 hours, wash them with Phosphate Buffered Saline (PBS).Next, they were treated again with ultrasonic cleaner three times for 10 minutes each.After washing, gradient dehydration was carried out in 50%, 70%, 80%, 90%, 95% and 100% alcohol solution, each gradient for 10 minutes, in 100% ethanol solutions dehydrated for 30 minutes and were dried in critical point dried (SCD-350M, Shianjia, Beijing, China) using CO2 as transfer fluid.Last, gold-sputtered for 1 min with gold (GVC-1000, GEVEE-TECH, Beijing, China).Observations of antennae and taken photos using a scanning electron microscope (SEM3200, Ciqtek, Hefei, China) with the acceleration voltage set as 10-20 kV.

Gross Morphology of Antennae
The antennae of C. nebulosa is filiform, and it is composed of three segments: a scape, a pedicel, and flagella (Fig 1).Female antennae are about two-thirds as long as male antennae.The entire length and width of antennae is 12.24 ± 8.61 mm and 165.62±27.99mm in males (n = 4), and 8.34 ± 7.75 mm and 89.34±21.27mm in females (n = 4).In the 2/3 distal part of flagella, the flagellomere of male resembles a cone slightly, the distal part of flagellomere is enlarged and the base is narrow, and the narrowing degree is variational in different flagellomere, but the flagellomere of female are nearly equal-diameter barrel-shaped.Different parts of antennae differ in shape, type and distribution of sensilla.There was no difference between male and female pedicel (Fig 3).

Grooved peg sensilla
The grooved peg sensilla are short, there were significant differences in male and female (Table 1).
The male are 8-11 μm long with a diameter of about 2-3 μm and the female are 5-10 μm long with a diameter of about 2-2.5 μm, stand in a small shallow depression (Figs 5c, 5d).Also have difference in distribution.The grooved peg sensilla in C. nebulosa occur on all faces of the entire flagellum in males (Fig 5a), but individual grooved peg sensilla only appears on the female antennae at least after the 20th node of the flagellum, gradually increasing at the distal antennal flagellomeres (Fig 5b).The outer surface of the cuticle is sculptured with many longitudinal grooves along most of its length and has a smooth surface at its basal part.This sensilla has different external shapes, some slightly curved, some straight (Figs 5c, 5d).There was no significant difference in the shape of male and female grooved peg sensilla.

Basiconic sensilla
The basiconic sensilla have a rod-shaped hair, female sensillum length is slightly longer than male.
The male is 12-17 μm in length and 2.5-4 μm in width, the female is 15-20 μm in length and 2.5-3.5 μm in width.This type of sensillum has a smooth perforated surface and a non-flexible socket (Fig 6a ), no significant difference in shape between male and female.At present, this kind of sensilla is only observed at the base of the scape and the pedicel.

Campaniform sensilla
The campaniform sensilla have a structure of dome-like.Two subtypes of campaniform sensilla are found in the distal part of the pedicellus.The biggest difference between the two subtypes is that there were two very obvious ridges on the edges of both sides of ScaII (Fig 6e), while ScaI have smooth edges on both sides (Fig 6f).The dome of ScaI is about 8-8.5 μm long and 4-5 μm wide, and there is a small hole with a diameter of 0.5-1 μm in the middle.ScaII is about 5-6 μm long and 3-4 μm wide, and the diameter of the middle hole is 0.5-1 μm.

Chaetic sensilla
The chaetic sensilla are the longest sensilla found in C. nebulosa and they are inserted in a flexible socket with an articulated base (Fig 7a).A grooved surface with striations is observed along all structures (transversally and longitudinally) (Figs 7c, 7d).There are two different subtypes of chaetic sensilla on the scapus and pedicellus, with the third chaetic sensilla distribute on the flagellum.ScI was thin and straight overall (Fig 7a), distributed at the distal end of the scapus and pedicellus with a length between 50-65 μm and a base diameter between 4.5-5 μm; ScII bended and tapered from the top to base, the distal end is obviously curved (Fig 7b), the length of the ScII has significant differences between male and female (Table 1), the length is 70-75 μm in male and 45-60 μm in female; ScIII is distributed in antennal flagellum, and only in the distal part of each flagellomeres, its length varieed greatly, gradually growing from the base to the end of the flagellum, up to about 150 μm.

Coelocapitular sensilla
The coelocapitular sensilla possess a grooved cuticular apparatus protuberance from a central hole that stands in a round hollow of 7 μm in diameter rimmed by a cuticular edge, externally only the tip of

Sensillar Distribution
The sensillar distribution pattern varied along the antennal longitudinal axis and between the sexes.
In order to represent these variations precisely, we developed a new nomenclature to discriminate different parts of the flagellum.Instead of using the traditional terms 'proximal', 'medial', and 'distal', which refer to imprecise longitudinal localizations, we divided the flagellum into 5 parts from the most proximal (part i) to the most distal (part v) (Figs 1, 8).Each part was characterized as follows.8c, 8d).Based on this characteristic, we referred to flagellomeres #8/10 to #15/20 as the part ii.In this part, the length and width of the male flagellomeres decreased by about 50 μm and 15 μm respectively, and the decrement of length and width in females were similar to that of males.

Part iii (flagellomeres #15/20 to #30)
Trichoid began to occur at this part in both sexes and grooved peg sensilla began to appear in females (Figs 8e, 8f) and these were used as a distinct criterion to distinguish the part iii from the part ii.
A large number of grooved peg sensilla dominated on flagellomeres of male, trichoid sensilla were in the majority on those of females.

Discussion
In this article, we study the C. nebulosa, revealing the external morphology and distribution of antennal sensilla in both males and females.The antennal exhibiting sexual dimorphism of C. nebulosa is shown in the number of grooved peg sensilla on the flagellum and in the length of the antennea.
Based on the morphological observations, we find that the antennal sensilla of C. nebulosa are similar to T. aridifolia.

Types and function of antennal sensilla
Grooved peg sensilla has been reported for numerous insects [20][21][22][23][24][25].In C. nebulosa, the grooved peg sensilla are very short with spoke canals and no flexible socket, and are located on the flagella.The grooved peg sensilla have a double-walled cuticular apparatus and the sensilla have two unbranched sensory cilia, these characteristics are common in most insect species, this type of sensillum is olfactory [25,26].The grooved peg sensilla on the antennae of the male C. nebulosa appeared from the first flagellomere and densely distributed in all directions of the flagellum (Fig 4), while the grooved peg sensilla of female appeared after the 15th flagellomere and the number was small.This trait is related to the need for the males to detect sex pheromones in the air [26,27].
The basiconic sensilla have a non-flexible socket, the base contains 2 sensory neurons and the cilia ramified in the hair lumen of the sensilla into 2 to several branches [18].These features similar to basiconic sensilla in other insect species, are related to olfaction and seem conserved among the different insect species [28][29][30][31].This type of sensilla is distributed in the base of the scape and pedicel.
We speculate that the uniqueness of the location of this sensilla may be perceived the precise angle of activity in each direction (except for the ventral view) and reduced friction.Through observing the behavior of praying mantis, the amplitude of antennal activity towards the ventral surface is the slightest, which may be the reason why this sensilla was not found on the ventral side.The position of the basiconic sensilla on the inside and outside of the scapus and pedicel is complementary, this complementarity can make the mantis antennae swing flexibly in multiple directions.
In C. nebulosa, trichoid sensilla are numerous.There are no significant differences between male and female.Trichoid sensilla arising from a shallow pore directly in the cuticle with a non-flexible socket are recognized as olfactory sensilla, it is usually considered to function as primary sex pheromone detectors, but some trichoid distributed in the scapus and pedicellus are considered mechanoreceptive sensilla [28,[32][33][34][35][36].Trichoid sensilla of C. nebulosa are mainly appeared after about 15 flagellomere and C. nebulosa have longer trichoid sensilla in male, probably because it need to detect female-emitted sex pheromones more efficiently [33,37,38].
The campaniform sensilla is considered a mechanoreceptor, it usually found on the wings of many insects [39][40][41][42].This type of sensillum is thought to detect general stresses in the cuticle possibly with some directional sensitivity [43,44].Campaniform sensilla distributed distal part of the pedicel in C.
nebulosa.It has a structure of dome-like, with a hole in the middle, it may be to sense the precise angle of antennal movement.According to its external shape, the campaniform sensilla was classified to two subtypes (Figs 6e, 6f), but the difference of their functions is still unclear.
Chaetic sensilla have been reported in hemiptera and diptera [45][46][47][48].They are the most widely distributed sensilla found on antennae of C. nebulosa, occurring on the scape, pedicel, and all flagellomeres.Its length gradually grows from the base to the end of the flagellum.According to the shape and basal width of the chaetic sensilla, we classified them into three subtypes.The chaetic sensilla are circularly distributed in a single line on each flagellomere in mantis with a strong and long-grooved cuticular apparatus and a flexible socket.In other insects have such structure and is considered to be mechano-and chemosensitive [49,50].So did the C. nebulosa.slightly, the distal part is enlarged and the base is narrow, and coefficient of variation scale.The base and end of the female antennal flagellomere are almost equal in width.The sensilla on the antennae of male and female praying mantis also exhibit some differences.ScⅢ and Sgp of male and female exhibit significant differences, the length and basal width of the two sensillum in male are larger than females (Table 1).These may be more advantageous for males to locate females more quickly.No significant differences in length and basal width are found between sexes with in St and ScⅡ (Table 1).
Females have slightly longer Sb in the scape than males, and they need to wave their antennae to sense surroundings to secure the location of spawning.

A comparison of antennae between C. nebulosa and T. aridifolia
A previous research have discussed in detail the types and distribution of sensilla on the antennae of T. aridifolia [19].Comparing the antennae of the C. nebulosa and the T. aridifolia, the male of the two species have differences slightly in the external morphology, while the female have no significant differences except for length.The male antennal flagellomere of the C. nebulosa is conical slightly, the distal part is enlarged and the base is narrow, and coefficient of variation scale (Figs 1, 4c, 4d, 8g, 8i).
While the base and distal part of the male antenna flagellomeres of the T. aridifolia are almost equal in width [19].The base and distal part of the flagellomeres of the female antennae of both species are also almost equal in width.In the overall length, the T. aridifolia antenna is longer than C. nebulosa, and the number of flagellomeres is also more numerous.
According to the different distribution of sensilla observed under SEM, Carle and Toh et al.
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Fig 1 .
Fig 1. Scanning electron microscopy images of male and female C. nebulosa.(a) Male antenna; (b) Female antennae.Part i-v in Fig 1 corresponds to part i-v in Fig 7. Sc, scape; Pe, the pedicel; Fl, flagellum.

Fig 4 .
Fig 4. SEM micrographs showing the pedicel of adult male C. nebulosa right antenna.(a) High magnification image of the first flagellomere of flagellum; (b) High magnification image of flagellomeres near the base; (c) High magnification image of the middle flagellomeres; (d) High magnification image of the apical flagellomeres.

Fig 5 .
Fig 5. Distribution and details of grooved peg sensilla.(a) Distribution of male grooved peg sensilla on the flagellum.(b) A Distribution of female grooved peg sensilla on the flagellum.(c,d) Different external shapes of grooved peg sensilla on the flagellum.Scale bars: I = 100 μm, II = 80 μm, III = 1 μm.
the sensilla is visible; occurring in low numbers and density on the flagellum (Fig 7f).A possible subtype with a larger central apparatus appears on the flagellum (Fig 7h).

Fig 7 .
Fig 7. Antennal sensilla of adult C. nebulosa.(a) ScI.(b) ScII.(c) Morphology of ScIII at the base of the flagellomere.(The arrow points to the groove on the chaetic sensilla) (d) Morphology of ScIII at the middle of the flagellomere, sensory length significantly increases.(e) Morphology of ScIII at the distal end of the flagellomere.(f) Sco.(g) and (h) A subtype of Sco.Positions of images (h) are indicated by lettered rectangles given in (g).Sc, chaetic sensilla; Sco, coelocapitular sensilla.

3. 3 . 1 .
Part i (flagellomeres #1 to #8/10) In the part i, between flagellomeres #1 and #8/10, chaetic sensilla were distributed along a single circular line perpendicular to the antennal axis in a distal region of each flagellomere in both sexes.The first clear sexual differences appeared to the flagellomere #1.In male C. nebulosa, grooved peg sensilla appeared from the first flagellomere of the flagellum, densely distributed on all sides of the flagellum (Fig 8a), exist until the end of the flagellum.Such sensilla were not observed in females in this part (Fig 8b).In contrast to females, which did not present this type of sensilla, males possessed a large number of grooved peg sensilla on each flagellomere from the flagellomere #1.3.3.2.Part ii (flagellomeres #8/10 to #15/20) Although the length of flagellomeres increased gradually and distally, it suddenly reduced at the vicinity of flagellomeres #8 to #10 (Figs

3. 3 . 4 .
Part iv(flagellomeres #30 to #40/45) In this part, trichoid sensilla number increased and they were distributed almost throughout the flagellum in both sex (Figs 8g, 8h), and the length increased slightly.The number of grooved peg sensilla was also significantly increased, and it was distributed in two thirds of the flagellomeres (Fig 8h).The change of chaetic sensilla sensilla in this part is mainly reflected in the length.The length of females increased by about 20 μm, and that of males can reach up to about 40 μm.In the part iii, the length of the chaetic sensilla was significantly different in sex, reaching 141.3±14.87μm in males and 54.26±2.75μm in females.3.3.5.Part v (flagellomeres40/45 to the distal end)Although the grooved peg sensilla were dominant on the flagellomeres from the parts i to iⅤ in males, both sexes presented a similar pattern of sensillar distribution in the part v (Figs 8i, 8j).That is, the sexual dimorphism in sensillar distribution disappeared in the part v.

Fig 8 .
Fig 8. Variation and sexual dimorphism in sensillar distribution on the flagellum.(a) The first flagellomere of male C. nebulosa in part i.(b) The first flagellomere of female C. nebulosa in part i. (c) The part ii of the male C. nebulosa suddenly becomes shorter.(d) The part ii of female C. nebulosa same as male.(e) The part iii of male C. nebulosa.(Inside circle is the trichoid sensilla).(f) The part iii of female C. nebulosa.(g) The part iv of male C. nebulosa.(h) The part iii of female C. nebulosa.(i) The part iv of male C. nebulosa.(j) The part v of female C. nebulosa.Inside circle is the trichoid sensilla and grooved peg sensilla in the box.

4 . 4 .
divided the antennal flagellum of T. aridifolia into six parts[19].But the antennal flagellum of C. nebulosa is revealed with five different parts.The difference is reflected in the distribution of grooved peg sensilla.The grooved peg sensilla of the male C. nebulosa appear from the first part of the antenna (Figs4a-d); But only chaetic sensilla are distributed in the first part of the male T. aridifolia, the grooved peg sensilla began to appear in the second part.The closer to the distal region, the more grooved peg sensilla.This is the reason why C. nebulosa is divided into five parts and T. aridifolia is divided into six parts.We speculate that the difference in the number and distribution of grooved peg sensilla may have a lot to do with the different living environment of T. aridifolia and C. nebulosa.The T. aridifolia is widely distributed, inhabiting the open space for gardens, low shrubs, grass, it is also common in the low altitude areas on the mountains.The C. nebulosa usually lives in the tropical rainforest with warm and humid climate.The C. nebulosa living environment is relatively complex.In the long process of evolution, it needs more sensilla to sense the surrounding environment.In addition to the grooved peg sensilla, the trichoid sensilla on the flagellum of the C. nebulosa are also more than the T. aridifolia.Compared with the coelocaputilar sensilla of the T. aridifolia and C. nebulosa, there is a new subtype of the coelocaputilar sensilla of the C. nebulosa.A hole was found at the distal end of the flagellum in a female C. nebulosa, and there was a structure in the hole similar to the central protrusion of the coelocaputilar sensilla, but it is thicker than common coelocaputilar sensilla on the antennae of praying mantis.The coelocaputilar sensilla have an external structure characteristic of hygroreceptive sensilla[52,53].Thus this new subtype may also be related to the living environment of the C. nebulosa.The structures discovered above are only observed on individual mantis antennae, which is accidental.More detailed conclusions need to observe more mantis and conduct more in-depth research.The way in which the flagellomeres divides Previous research in T. aridifoli, the flagellum is divided into part α-ζ(the part near the base is part α) based on sensilla distribution[19].The chaetic sensilla were distributed on two circular lines around a distal region and a medial region on each flagellomere in part β of the T. aridifolia.But in the part γ, the sudden reduction of the length of flagellomeres was accompanied by a reduction in the number of circular lines of chaetic sensilla from a double to a single line[19].After that, Carle et al. made further research on the antennae of the T. aridifolia, and explored the development mode of its antennae.The research revealed that the flagellum of the antennae began to split from two places, one was the first flagellomere, where multiple line of chaetica sensillum distributed, the other is the part β with double line chaetica sensillum[18].Based on this finding, we focused on the distribution of chaetica sensillum on C. nebulosa antennae and found no such feature of double line chaetic sensillum on their entire antennae.So we speculate it may belong to the generic sign of T. aridifolia.And the way of division may be different in this part.No matter the C. nebulosa, S. maculata or T. aridifolia, they all have multiple-circle of chaetica sensillum are distributed on the first flagellomere, which may be used as meristematic section.

Table 1 . Length and basal width of sensillum type of C. nebulosa Zheng Type Length (μm) t-Test Basal Width (μm) t-Test Males Female Males Female
Male C. nebulosa have thicker and longer antennae than female, showing significant differences in the length and width of the flagellomere.And the shape of the antennae of male C. nebulosa is different from females.The shape of male flagellomere is conical